Lec 10 - Mechanics of Body Movement and Skeletal Muscle Reflexes

How is skeletal muscle classified?

By speed and fatigue resistance

Slow-twitch fibers (ST or type I)?

Rely primarily on oxidative phosphorylation

Fast-twitch fibers: general properties?

Develop tension faster (split ATP more rapidly); pump Ca2+ into the sarcoplasmic reticulum more rapidly

Fast-twitch oxidative-glycolytic (FOG or type IIA)?

Use oxidative and glycolytic metabolism

Fast-twitch glycolytic (FG or type IIB/X)?

Rely primarily on anaerobic glycolysis

What does myoglobin do?

Binds oxygen in muscle so it is readily available for aerobic processes

Which fibers have more myoglobin?

Oxidative fibers

Fig 12.14: Slow-twitch oxidative fibers?

Large amounts of red myoglobin, numerous mitochondria, extensive capillary blood supply; smaller diameter, darker color (myoglobin), fatigue-resistant (e.g., leg muscle)

Fig 12.14: Fast-twitch glycolytic fibers?

Larger diameter, pale color, easily fatigued (e.g., eye muscles); fewer capillaries and mitochondria

Table 12.2: Speed of maximum tension?

Type I slowest; IIA intermediate; IIB/IIX fastest

Table 12.2: Myosin ATPase activity?

Type I slow; IIA fast; IIB/IIX fast

Table 12.2: Diameter?

Type I small; IIA medium; IIB/IIX large

Table 12.2: Contraction duration?

Type I longest; IIA short; IIB/IIX short

Table 12.2: Ca2+-ATPase activity in SR?

Type I moderate; IIA high; IIB/IIX high

Table 12.2: Endurance?

Type I fatigue resistant; IIA fatigue resistant; IIB/IIX easily fatigued

Table 12.2: Use?

Type I most used (posture); IIA standing, walking; IIB/IIX least used (jumping; quick, fine movements)

Table 12.2: Metabolism?

Type I oxidative, aerobic; IIA glycolytic but more oxidative with endurance training; IIB/IIX glycolytic, more anaerobic than IIA

Table 12.2: Capillary density?

Type I high; IIA medium; IIB/IIX low

Table 12.2: Mitochondria?

Type I numerous; IIA moderate; IIB/IIX few

Table 12.2: Color?

Type I dark red (myoglobin); IIA red; IIB/IIX pale

Table 12.2: Muscle color names?

Type I and IIA = red muscle; IIB/IIX = white muscle

How does resting fiber length affect contraction?

Sarcomeres contract with optimum force at optimum length (neither too long nor too short) before contraction begins

Tension and crossbridges?

Tension generated is directly proportional to the number of crossbridges

Fig 12.15: Too much or too little thick/thin filament overlap?

Results in decreased tension

Fig 12.15: Length-tension numbers?

Optimal resting length about 2.0-2.3 micrometers (100% tension); tension falls to 0 at 1.3 micrometers (decreased length) and 3.7 micrometers (increased length)

What increases force of contraction?

Summation

Summation?

Stronger contraction when the muscle does not relax completely between action potentials

Tetanus?

Maximal contraction

Fig 12.16a: Single twitches?

Muscle relaxes completely between stimuli

Fig 12.16b: Summation?

Stimuli closer together do not allow muscle to relax fully (summed twitches)

Fig 12.16c: Unfused (incomplete) tetanus?

Stimuli are far enough apart to allow slight relaxation between stimuli; tension reaches maximum with ripples

Fig 12.16d: Complete (fused) tetanus?

Muscle reaches steady maximum tension; if muscle fatigues, tension decreases rapidly despite continuing stimuli

What is a motor unit?

One motor neuron and its muscle fibers (all fibers it innervates)

What determines contraction force?

The types and numbers of motor units

Recruitment?

Recruitment of additional motor units increases contraction force

Asynchronous recruitment?

Alternate recruitment of motor units, which helps avoid fatigue

Fig 12.17: Motor units in one muscle?

A muscle may have many motor units of different fiber types (Neurons 1, 2, 3 in the spinal cord; motor nerve to muscle fibers)

Isotonic vs isometric contractions?

Isotonic contractions move loads; isometric contractions create force without movement

Concentric action?

Isotonic contraction in which muscle shortens

Eccentric action?

Isotonic contraction in which muscle lengthens

Isometric contractions and series elastic elements?

Sarcomeres shorten while elastic elements stretch, resulting in little change in overall length

Fig 12.18a: Isotonic contraction?

Muscle contracts, shortens and creates enough force to move the load (e.g., 20 kg load moves; tension reaches force required to move load)

Fig 12.18b: Isometric contraction?

Muscle contracts but does not shorten; force created (about 25 kg) cannot move the 30 kg load

Fig 12.19: Components of muscle?

Contractile components (sarcomeres; gear and ratchet) and elastic components (spring)

Fig 12.19: Isometric contraction?

Sarcomeres shorten generating force, but elastic elements stretch, so muscle length stays the same

Fig 12.19: Isotonic contraction?

Sarcomeres shorten more but, because elastic elements are already stretched, the muscle shortens

Lever?

A rigid bar that pivots around a point called a fulcrum

What forms levers and fulcrums in the body?

Bones form levers; flexible joints form fulcrums

Fig 12.20a: Forearm lever like?

A fishing pole: fulcrum at one end, load at the other end, force applied between them

Fig 12.20b: Forearm lever parts?

Fulcrum = elbow joint; load = gravity acting on mass of forearm and hand; biceps muscle applies force

Fig 12.20c: Biceps force calculation?

Biceps inserts 5 cm from fulcrum; forearm weight 2 kg at 15 cm. Rotational force up = down: biceps force x 5 cm = 2 kg x 15 cm; biceps force = 30 kg-cm / 5 cm = 6 kg

Fig 12.20d: What does the arm amplify?

Speed of movement of the load; biceps insertion is close to the fulcrum, so 1 cm of biceps shortening moves the hand 5 cm upward

1st class lever?

Fulcrum between effort and load (e.g., head/neck, scissors)

2nd class lever?

Load between effort and fulcrum (e.g., calf raising foot, wheelbarrow)

3rd class lever?

Effort between fulcrum and load (e.g., biceps/forearm, tweezers)

Fig 12.21: Load-velocity relationship?

Velocity of shortening decreases as load on the muscle increases

Fig 12.21 graph questions: Where is contraction isometric / at maximum velocity?

Isometric: point E (zero velocity at greatest load). Maximum velocity: point A (lowest load)

Golgi tendon organs (GTO): location?

Found at the junction of tendons and muscle fibers

GTO structure?

Free nerve endings that wind between collagen fibers inside a connective tissue capsule

GTO and tendons?

When a muscle contracts, its tendons act as a series elastic element during the isometric phase

GTO function?

Sends sensory information to the CNS

Fig 13.2a: What does the GTO link?

The muscle and the tendon; sensory nerve endings interwoven among collagen fibers

Fig 13.2a: When does the GTO sensory neuron fire?

When the muscle contracts and pulls the collagen fibers of the tendon tight

Fig 13.2a: Extrafusal muscle fibers?

Normal contractile fibers

Muscle spindles?

Stretch receptors that encode signals about muscle length and changes in length

Muscle spindle capsule?

Encloses a group of small muscle fibers known as intrafusal fibers

Muscle spindle innervation?

From gamma motor neurons

Stretch reflex?

A contraction response to muscle stretch

Alpha-gamma coactivation?

Simultaneous activation of alpha and gamma motor neurons

Alpha motor neurons in coactivation?

Fire; muscle shortens; tension released

Gamma motor neurons in coactivation?

Fire; intrafusal fibers contract; maintain stretch

Result of alpha-gamma coactivation?

Spindle remains active

Fig 13.2b: Where are muscle spindles?

Buried among the extrafusal fibers; send information about muscle stretch to the CNS

Fig 13.2b: Alpha motor neuron?

Innervates extrafusal muscle fibers

Fig 13.2b: Gamma motor neurons?

From CNS; innervate intrafusal fibers and cause their contraction

Fig 13.2b: Spindle sensory neurons and central region?

Tonically active sensory neurons send info to CNS; central region lacks myofibrils

Fig 13.4: Alpha-gamma coactivation steps?

1) Alpha and gamma motor neurons fire. 2) Muscle and intrafusal fibers both contract. 3) Stretch on centers of intrafusal fibers unchanged; afferent firing rate stays constant

Fig 13.4: Why does the firing rate stay constant?

Gamma motor neurons innervate fibers at the ends of spindles; intrafusal fibers do not slacken when the muscle shortens; keeps spindle stretched and maintains function

Learning outcomes (part 1)?

Discuss causes of muscle fatigue; discuss differences between slow-twitch, FOG and FG fibers; explain how muscle length influences force; distinguish summation and types of tetanus; define motor unit and graded contractions

Learning outcomes (part 2)?

Compare isometric and isotonic contractions; describe bones and muscles forming fulcrums and levers

Learning outcomes (part 3, reflexes)?

Diagram steps of a skeletal muscle reflex (alpha motor neuron, proprioceptor, extrafusal fibers, muscle tone); diagram a stretch reflex with alpha-gamma coactivation; explain patellar tendon reflex (monosynaptic stretch reflex, reciprocal inhibition, myotatic unit); diagram a flexion reflex and crossed extensor reflex

Key words (fibers and tension)?

slow-twitch (ST or type I) fibers, fast-twitch oxidative-glycolytic (FOG or type IIA) fibers, fast-twitch glycolytic (FG or type IIB/IIX) fibers, myoglobin, red muscle, white muscle, summation, tetanus, incomplete (unfused) tetanus, complete (fused) tetanus, motor unit, recruitment, asynchronous recruitment

Key words (contractions and reflexes)?

isotonic contraction, lengthening (eccentric) contraction, isometric contraction, static contraction, series elastic elements, proprioceptors, joint receptors, alpha motor neurons, extrafusal muscle fibers, muscle tone, Golgi tendon organ (GTO), muscle spindles, capsule, intrafusal fibers, gamma motor neurons, stretch reflex, alpha-gamma coactivation